The invention concerns esterification catalysts and in particular esterification catalysts which comprise novel organotitanium, organozirconium or organoaluminium compounds.
Organotitanium compounds and, in particular, titanium alkoxides or orthoesters are known as catalysts for esterification processes. During the esterification, these compounds are frequently converted to insoluble compounds of titanium which result in a hazy product The presence of a haze is a particular disadvantage in polyesters which have a high viscosity and/or high melting point and are therefore difficult to filter. Furthermore, many organotitanium compounds which are effective catalysts in the manufacture of polyesters such as polyethylene terephthalate are known to produce unacceptable yellowing in the final polymer. Our co-pending application, published as GB 2 314 081 relates to an esterification process in which these problems are partially solved but there is still a need for a catalyst which induces little or no yellowing in a polyester produced using the catalyst.
It is an object of the present invention to provide an improved catalyst for process for preparing esters.
According to the invention, an organometallic compound suitable-for use as a catalyst for the preparation of an ester comprises the reaction product of an orthoester or condensed orthoester of titanium, zirconium or aluminium, an alcohol containing at least two hydroxyl groups, an organophosphorus compound containing at least one Pxe2x80x94OH group and a base.
Also according to the invention, a process for the preparation of an ester comprises carrying out an esterification reaction in the presence of a catalyst comprising the reaction product of an orthoester or condensed orthoester of titanium, zirconium or aluminium, an alcohol containing at least two hydroxyl groups, an organophosphorus compound containing at least one Pxe2x80x94OH group and a base.
In a further embodiment the organometallic compound suitable for use as a catalyst in an esterification process comprises the reaction product of an orthoester or condensed orthoester of titanium, zirconium or aluminium, an alcohol containing at least two hydroxyl groups, an organophosphorus compound containing at least one Pxe2x80x94OH group, a base and a 2-hydroxy carboxylic acid.
The organometallic compound of the invention is the reaction product of a titanium, zirconium or aluminium orthoester or condensed orthoester, an alcohol containing at least two hydroxyl groups, an organophosphorus compound containing at least one Pxe2x80x94OH group and a base. Preferably, the orthoester has the formula M(OR)4 or AJ(OR)3 where M is titanium or zirconium and R is an alkyl group. More preferably R contains 1 to 6 carbon atoms and particularly suitable orthoesters include tetraisopropoxy titanium, tetra-n-butoxy titanium, tetra-n-propoxy zirconium, tetra-n-butoxy zirconium and tetra-iso-butoxy aluminium.
The condensed orthoesters suitable for preparing the compounds of this invention are typically prepared by careful hydrolysis of titanium, zirconium or aluminium orthoesters. Titanium or zirconium condensed orthoesters are frequently represented by the formula
R1O[M(OR1)2O]nR1
in which R1 represents an alkyl group and M represents titanium or zirconium. Preferably, n is less than 20 and more preferably is less than 10. Preferably, R1 contains 1 to 12 carbon atoms, more preferably, R1 contains 1 to 6 carbon atoms and useful condensed orthoesters include the compounds known as polybutyl titanate, polyisopropyl titanate and polybutyl zirconate.
Preferably, the alcohol containing at least two hydroxyl groups is a dihydric alcohol and can be a 1,2-diol such as 1,2-ethanediol or 1,2-propanediol, a 1,3-diol such as 1,3-propanediol, a 1,4-diol such as 1,4-butanediol, a diol containing non-terninal hydroxyl groups such as 2-methyl-2,4pentanediol or a dihydric alcohol containing a longer chain such as diethylene glycol or a polyethylene glycol. Preferred dihydric alcohols are 1,2-ethanediol and diethylene glycol. The organometallic compound can also be prepared from a polyhydric alcohol such as glycerol, trimethylolpropane or pentaerythritol.
Preferably, the organometallic compound useful as a catalyst is prepared by reacting a dihydric alcohol with an orthoester or condensed orthoester in a ratio of from 1 to 16 moles of dihydric alcohol to each mole of titanium, zirconium or aluminium. More preferably, the reaction product contains 2 to 12 moles of dihydric alcohol per mole of titanium, zirconium or aluminium and most preferably 4 to 8 moles dihydric alcohol per mole of titanium, zirconium or aluminium.
The organophosphorus compound which contains at least one Pxe2x80x94OH group can be selected from a number of organophosphorus compounds including phosphates, pyrophosphates, phosphonates, phosphinates and phosphites.
Preferably, the organophosphorus compound is a substituted or unsubstituted alkyl phosphate, a substituted or unsubstituted aryl phosphate or a phosphate of an alkylaryl glycol ether or an alkyl glycol ether. Preferred compounds include monoalkyl acid phosphates and dialkyl acid phosphates and mixtures of these. Particularly convenient organophosphorus compounds are the compounds commercially available as alkyl acid phosphates and containing, principally, a mixture of mono- and di-alkyl phosphate esters. When an alkyl phosphate is used, the organic group preferably contains up to 20 carbon atoms, more preferably up to 8 carbon atoms and, most preferably, up to 6 carbon atoms. When alkylaryl or alkyl glycol ether phosphates are used the carbon chain length is preferably up to 18 carbon atoms and, more preferably, 6 to 12 carbon atoms. Particularly preferred organophosphorus compounds include butyl acid phosphate, polyethylene glycol phosphates and aryl polyethylene glycol phosphates.
The amount of organophosphorus compound present in the reaction product of the invention is usually in the range 0.1 to 4.0 mole of phosphorus to 1 mole of metal (titanium, zirconium or aluminium), preferably in the range 0.1 to 2.0 mole phosphorus to 1 mole metal and most preferably in the range 0.1 to 1.0 mole phosphorus to 1 mole metal.
A base is also used in preparing the reaction product of the invention. The base is generally an inorganic base and suitable bases include sodium hydroxide, potassium hydroxide and ammonium hydroxide, but organic bases such as tetrabutyl ammonium hydroxide or choline hydroxide [trimethyl-(2-hydroxyethyl)ammonium hydroxide] can also be used. Usually, the amount of base used is in the range 0.1 to 4.0 mole base per mole of metal (titanium, zirconium or aluminium). The preferred amount is in the range 0.1 to 2.0 mole base per mole of metal and, frequently, the amount of base present is in the range 0.1 to 1.0 mole base per mole of titanium, zirconium or aluminium.
When 2-hydroxy carboxylic acids are used to prepare the products of the invention, preferred acids used include lactic acid, citric acid, malic acid and tartaric acid. Some suitable acids are supplied as hydrates or as aqueous mixtures and can be used in this form. When a 2-hydroxy acid is present, the preferred molar ratio of acid to titanium, zirconium or aluminium in the reaction product is 0.5 to 4 moles per mole of titanium, zirconium or aluminium. More preferably the catalyst contains 1.0 to 3.5 moles of 2-hydroxy acid per mole of titanium, zirconium or aluminium.
The organometallic compound can be prepared by mixing the components (orthoester or condensed orthoester, alcohol containing at least two hydroxyl groups, organophosphorus compound and base) with removal, if desired, of any by-product, (e.g. isopropyl alcohol when the orthoester is tetraisopropoxytitanium), at any appropriate stage. In one preferred method the orthoester or condensed orthoester and a dihydric alcohol are mixed and, subsequently, a base is added, followed by the organophosphorus compound. When a 2-hydroxy carboxylic acid is also present in the reaction product, this is usually added to the orthoester or condensed orthoester before the organophosphorus compound is added. Altematively, all or part of the 2-hydroxy carboxylic acid can be neutralised with the base and the resulting salt added to the other components of the reaction mixture, including, if desired, a further portion of the base.
The esterification reaction of the process of the invention can be any reaction by which an ester is produced. The reaction may be (i) a direct esterification in which a carboxylic acid or its anhydride and an alcohol react to form an ester or (ii) a transesterification (alcoholysis) in which a first alcohol reacts with a first ester to produce an ester of the first alcohol and a second alcohol produced by cleavage of the first ester or (iii) a transesterification reaction in which two esters are reacted to form two different esters by exchange of alkoxy radicals. Direct esterification or transesterification can be used in the production of polymeric esters and a preferred process of the invention comprises a polyesterification process. Many carboxylic acids and anhydrides can be used in direct esterification including saturated and unsaturated monocarboxylic acids and anhydrides of such acids such as stearic acid, isostearic acid, capric acid, caproic acid, palmitic acid, oleic acid, palmitoleic acid, triacontanoic acid, benzoic acid, methyl benzoic acid, salicylic acid and rosin acids such as abietic acid, dicarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, sebacic acid, adipic acid, azelaic acid, succinic acid, fumaric acid, maleic acid, naphthalene dicarboxylic acid and pamoic acid and anhydrides of these acids and polycarboxylic acids such as trimellitic acid, citric acid, trimesic acid, pyromellitic acid and anhydrides of these acids. Alcohols frequently used for direct esterification include aliphatic straight chain and branched monohydric alcohols such as butyl, pentyl, hexyl, octyl and stearyl alcohols, dihydric alcohols such as 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol and 1,6-hexanediol and polyhydric alcohols such as glycerol and pentaerythritol. A preferred process of the invention comprises reacting 2-ethylhexanol with phthalic anhydride to form bis(2-ethylhexyl)phthalate.
The esters employed in an alcoholysis reaction are generally the lower homologues such as methyl, ethyl and propyl esters since, during the esterification reaction, it is usual to eliminate the displaced alcohol by distillation. These lower homologue esters of the acids suitable for direct esterification are suitable for use in the transesterification process according to the invention. Frequently (meth)acrylate esters of longer chain alcohols are produced by alcoholysis of esters such as methyl acrylate, methyl methacrylate, ethyl acrylate and ethyl methacrylate. Typical alcohols used in alcoholysis reactions include butyl, hexyl, n-octyl and 2-ethyl hexyl alcohols and substituted alcohols such as dimethylaminoethanol.
When the esterification reaction is a transesterification between two esters, generally the esters will be selected so as to produce a volatile product ester which can be removed by distillation.
As mentioned hereinbefore, polymeric esters can be produced by processes involving direct esterfication or transesterification and a particularly preferred embodiment of the esterification process of the invention is a polyesterification reaction in the presence of the catalyst described hereinbefore. In a polyesterification reaction polybasic acids or esters of polybasic acids are usually reacted with polyhydric alcohols to produce a polymeric ester. Linear polyesters are often produced from dibasic acids such as those mentioned hereinbefore or esters of said dibasic acids and dihydric alcohols. Preferred polyesterification reactions according to the invention include the reaction of terephthalic acid or dimethyl terephthalate with 1,2-ethanediol (ethylene glycol) to produce polyethylene terephthalate or with 1,4-butanediol (butylene glycol) to produce polybutylene terephthalate or reaction of naphthalene dicarboxylic acid with 1,2-ethanediol to produce polyethylene naphthalenate. Other glycols such as 1,3-propanediol, 1,6-hexanediol and polyhydric alcohols such as glycerol, trimethylolpropane and pentaerythritol are also suitable for preparing polyesters.
The esterification reaction of the invention can be carried out using any appropriate, known technique for an esterification reaction.
A typical process for the preparation of polyethylene terephthalate comprises two stages. In the first stage terephthalic acid or dimethyl terephthalate is reacted with 1,2-ethanediol to form a prepolymer and the by-product water or methanol is removed. The prepolyrner is subsequently heated in a second stage to remove 1,2-ethanediot and form a long chain polymer. Either or both these stages may comprise an esterification process according to this invention.
In direct esterification the acid or anhydride and an excess of alcohol are typically heated, if necessary in a solvent, in the presence of the catalyst. Water is a by-product of the reaction and this is removed, as an azeotrope with a boiling mixture of solvent and/or alcohol. Generally, the solvent and/or alcohol mixture which is condensed is immiscible with water which is therefore separated before solvent and/or alcohol are returned to the reaction vessel. When reaction is complete the excess alcohol and, when used, solvent are evaporated. In view of the fact that the catalysts of the invention do not normally form insoluble species, it is not generally necessary to remove them from the reaction mixture, as is frequently necessary with conventional catalysts. A typical direct esterification reaction is the preparation of bis(2-ethylhexyl) phthalate which is prepared by mixing phthalic anhydride and 2-ethyl hexanol. An initial reaction to form a monoester is fast but the subsequent conversion of the monoester to diester is carried out by refluxing in the presence of the catalyst at a temperature of 180-200xc2x0 C. until all the water has been removed. Subsequently the excess alcohol is removed.
In an alcoholysis reaction, the ester, first alcohol and catalyst are mixed and, generally, the product alcohol (second alcohol) is removed by distillation often as an azeotrope with the ester. Frequently it is necessary to fractionate the vapour mixture produced from the alcoholysis in order to ensure that the second alcohol is separated effectively without significant loss of product ester or first alcohol. The conditions under which alcoholysis reactions are carried out depend principally upon the components of the reaction and generally components are heated to the boiling point of the mixture used.
A preferred process of the invention is the preparation of polyethylene terephthalate. A typical batch production of polyethylene terephthalate is carried out by charging terephthalic acid and ethylene glycol to a reactor along with catalyst if desired and heating the contents to 260-270xc2x0 C. under a pressure of about 0.3 MPa. Reaction commences as the acid dissolves at about 230xc2x0 C. and water is removed. The product is transferred to a second autoclave reactor and catalyst is added, if needed. The reactor is heated to 285-310xc2x0 C. under an eventual vacuum of 100 Pa to remove ethylene glycol by-product. The molten product ester is discharged from the reactor, cooled and chipped. The chipped polyester may be then subjected to solid state polymerisation, if appropriate.
The amount of catalyst used in the esterification process of the invention generally depends upon the titanium, zirconium or aluminium content, expressed as Ti, Zr or Al, of the catalyst Usually the amount is from 30 to 1000 parts per million (ppm) of metal on weight of product ester for direct or transesterification reactions. Preferably, the amount is from 30 to 450 ppm of metal on weight of product ester and more preferably 50 to 450 ppm of metal on weight of product ester. in polyesterification reactions the amount used is generally expressed as a proportion of the weight of product polyester and is usually from 5 to 500 ppm expressed as Ti, Zr or Al based on product polyester. Preferably the amount is from 5 to 100 ppm expressed as Ti, Zr or Al based on product polyester.
The products of this invention have been shown to be effective catalysts for producing esters and polyesters at an economical rate without leading to haze in the final product and with a reduced amount of yellowing of polyesters in comparison to known catalysts. They have also been shown to be stable against precipitation from polyester products when aqueous base or phosphoric acid is added to such products.